Character model rigging error

I am currently porting a model from Grand theft auto 5 for use (Non commercially) in VR chat, I have previous experience with some models before but those were much easier to do as they were in perfect T pose.

Anyways the issue Im having here is this model when put into unity and in the bone configuration area, the shoulders are a bit off and I cant seem to figure out how to edit the mesh/bones in a way where it can perfect T pose without the shoulders being distorted in this way.

The model isnt in a perfect T pose as you can see from the blender screenshot and manually editing the models arms to be level isnt a very feasable thing to do (with my current experience and knowledge) the mesh isnt completely joined together (despite technically being just 1 mesh) the arms are not connected to the shoulders for some reason Im not sure why this is so but, I guess its a model made by Rockstar games soooo I guess I cant argue.

Anyone know a way that I can fix the shoulders so he looks normal while in perfect T pose form, the rest of the model is rigged great its just the shoulders that are having a problem really :L

Ive tried rotating/moving the shoulder bones around to see if it would make a difference when ported to Unity but every single time no matter what I do Unity always forces the models shoulders and shoulder bones in the positions as pictured.